Shedding light on non-equilibrium dynamics of a spin coupled to fermionic reservoir

When a quantum system is subjected to a quantum quench, i.e. a sudden change of some parameters of the Hamiltonian, its subsequent dynamics is governed by energy scales that become ever lower with increasing time: whereas the transient behavior right after the quench depends on high-energy excitations, the asymptotic long-time evolution is determined by low-lying excitations close to the final ground state. Thus, time- or frequency-resolved probes of the dynamics after a quantum quench offers insight into the nature of the system's eigenstates across the entire energy spectrum. We recently proposed that such a quantum quench for the single-impurity Anderson Hamiltonian can be induced by the sudden creation of an exciton in a quantum dot via optical absorption of an incident photon of definite frequency. The subsequent emergence of correlations between the spin degrees of freedom of the dot and a tunnel-coupled low-temperature Fermionic reservoir, ultimately leading to the Kondo effect, can be accurately mapped out through an optical absorption experiment. This experiment was recently carried out with semiconductor quantum dots coupled to a degenerate electron gas, demonstrating experimentally for the first time the optical signature of Kondo correlations. I will present the theory behind the resulting lineshape that is found to unveil three very different dynamical regimes, corresponding to short, intermediate and long times after the initial excitation, which are in turn described by the three renormalization group fixed points of the Anderson Model.